The present invention relates generally to data transmission in mobile communication systems and more specifically to methods and systems for facilitating transparent wireless relay using dual layer beam forming procedures.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants, handheld or laptop computers, and similar devices or other User Equipment (“UE”) that have telecommunications capabilities. In some embodiments, a UA may refer to a mobile, wireless device. The term “UA” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes.
In traditional wireless telecommunications systems, transmission equipment in a base station transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment, and a packet-based network that uses such equipment can be referred to as an evolved packet system (EPS). Additional improvements to LTE systems/equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UA with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UAs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UAs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UAs, deciding the transport channel to be used for each UA's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UAs through a control channel.
It is desirable to provide high data rate coverage for UAs serviced by a base station. Typically, only those UAs that are physically close to a base station can operate with a very high data rate, and to provide high data rate coverage over a large geographical area a large number of base stations are required. As the cost of implementing such a system can be prohibitive, research is being conducted on alternative techniques to provide wide area, high data rate service.
One promising technique for increasing rate coverage is to use relay nodes (RNs) to distribute data more evenly in a cell served by a particular base station. In general, an RN can be employed to increase signal strength within a cell when a poor direct link between a UA and a base station occurs. Among different types of relays, a transparent or type II relay is particularly useful because it is simple to implement and has a relatively low cost. A transparent relay employs RNs that do not have their own cell IDs and, in at least some cases, do not have their own sync channels and control channels. The transparent relay mainly helps an associated base station transmit data and the RN is transparent in the sense that a UA cannot distinguish if a received transmission is from a base station or an RN (i.e., the UA is unaware of the existence of the RN).
One approach to relay design is to have a relay help with only data re-transmissions. In such a system, the base station initially transmits data to the UA. If the initial transmission fails, one or multiple RNs help retransmit the data by transmitting the re-transmission signal to the UA at the same time as and using the same resources as the base station. The signals transmitted by the RN and the base station combine (i.e., superpose) in the air to provide a stronger signal and thus increase the chance of re-transmission success. In other systems, in addition to being used to help with re-transmission, RNs are also used to help with initial transmissions.
One other technique for increasing rate coverage is to use directional beams to transmit information from a base station or an RN to a UA under certain circumstances. To this end, by transmitting a beam toward a UA as opposed to generally broadcasting data, the strength of the transmitted signal can be increased appreciably. Stronger signals are easier for a UA to successfully receive. Base stations can typically communicate via various communication modes including broadcast modes and at least one directional beam forming mode (e.g., LTE Rel-8 transmission mode 7) and the mode used can be dynamically modified as a function of channel quality signal strength, etc.
A communication system may support various reference signals for the downlink and uplink to facilitate beamforming and other functions such as determining which of the several different communication modes should be used to communicate with a UA. A reference signal is a signal generated based on known data and may also be referred to as a pilot, preamble, training sequences, sounding reference signal, etc. A reference signal may be used by a receiver for various purposes such as channel estimation, coherent demodulation, channel quality measurement, signal strength measurement, etc. Exemplary reference signals include a cell-specific or common reference signal (CRS) that is sent by a base station to UAs within a cell and is used for channel estimating and channel quality measurement, a UA-specific or dedicated reference signal (DRS) that is sent by a base station to a specific UA within a cell which is used for demodulation of a downlink transmission, a sounding reference signal (SRS) sent by a UA and used by a base station for channel estimation and channel quality measurement and a demodulation reference signal sent by a UA and used by a base station for demodulation of an uplink transmission from the UA.
As the physical channels experienced by signals from RN(s) and the base station are different, separate reference signals (RS) may need to be transmitted from the base station and each relay. There are two options of RS transmission from a RN including the CRS and the DRS. To reduce interference with other UAs using CRS, in at least some cases, DRS is used for transparent relay.
Current LTE devices (e.g., Rel-8 devices) may use a single-layer beamforming (BF) mode (e.g., transmission mode 7) for scheduling UAs that need relay help. Here, for example, when an initial transmission fails, the RN helps with single layer beam forming re-transmission. However empirical evidence has shown that transparent single-layer BF modes, in some cases, may not bring much improvement in sector and cell edge throughput.
Another solution for increasing sector and cell edge throughput that is being considered for next generation LTE devices is dual-layer BF. In dual-layer systems, instead of forming a single beam for transmitting data to a UA, a base station generates first and second separate beams where each of the beams transmits different streams of data. In theory, by using two streams instead of one, the throughput to a UA should be increased appreciably. Currently there is no transmission scheme to support a dual-layer BF relay system.